Tuesday, May 17, 2016

Understanding the power - heart rate gap

Today’s Tantalus ride was intended first and foremost to be a recovery ride, but my legs felt good so I decided to get more out of it by practicing my Honu climbing pace. This was especially challenging because I forgot my Edge 800 head unit, so I had to go mostly by feel. I wear my Fenix2 on my wrist and was able to glance occasionally at power zone and heart rate zone. At first I wondered why my heart rate was so low, until I remembered the the Fenix2 only supports one set of zones and I have them set up for running, whereas the Edge 800 is configured for cycling. My run LTHR is 154, whereas my cycling LTHR is 140 (this is normal). It works out that the zone displayed this morning on the Fenix2 was roughly one too low. The data as displayed on Training Peaks is correct.

After staring at the results on Training Peaks for a time I decided to compare today’s data with Sunday’s race data. Found something interesting, and not entirely surprising. To get a more meaningful comparison I only used the climbing portion from today and compared that to the entire bike leg of the race. Using the totals from today would include too much downhill. The climbing portion of the ride and the entire race leg represent continuous nearly constant effort.

Honolulu Triathlon, May 15, entire bike leg (1:24)The thing that jumps out at me is how similar the heart rate numbers are, yet the power output on Sunday was much lower than today’s ride. Sunday’s ride was flat, while today’s was a long, steep climb. This is reflected in the cadence numbers; Sunday’s average was 81, today’s was 48. On Sunday I was not monitoring heart rate at all, only power and cadence, but I could feel my legs getting the burn that signals getting close to the lactate threshold. Sure enough, I was at 95% LTHR. Really cannot do any better than that over an hour and thirty minute time trial.

As good as that sounds, why was I only putting out 70% FTP on Sunday? Today my heart rate was 93% LTHR yet my average power was 94% FTP. I can think of four possible reasons. First, the excitement of the race. Call it nerves. Excitement will raise your heart rate. I was probably near LTHR just standing on the shore waiting to start! Second, cadence. I cannot spin up on the Tantalus climb. The long, slow pedal stroke is not quite the same as a fast spin, and since I do not get many opportunities to ride all out on flat roads my body is better adapted to climbing. (Hand me that polka dot jersey.) Third, fatigue. I did the bike leg on Sunday after a long, hard swim, and it lasted 50% longer than the Tantalus climb. But I did the first hour on Sunday at 72% FTP, and the Variability Index of 1.02 tells us I did not slow down at the end. Still, the swim could be the difference. Last but not least, speed. Like I already said, I rarely get to go all out for any length of time. Going twenty miles an hour down any highway on Oahu is to risk life and limb. Ford Island is good for this, but not the most convenient place to get to. My theory is that whenever I got up to 130 watts on Sunday my instincts told me to slow down.

I doubt that anyone could say for sure why the gap between power and heart rate was so much higher Sunday than today. One take away from all this is the need to monitor power and heart rate during the race. Forcing myself to hold a planned power could result in walking the run. As good a metric as power is, heart rate cannot be ignored.